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Twin boundaries (TBs) play an essential role in enhancing the mechanical, electronic and transport properties of polycrystalline materials. However, the mechanisms are not well understood. In particular, we considered that they may play an important role in boron rich boron carbide (B vr BC), which exhibits promising properties such as low density, super hardness, high abrasion resistance, and excellent neutron absorption. Here, we apply first-principles-based simulations to identify the atomic structures of TBs in B vr BC and their roles for the inelastic response to applied stresses. In addition to symmetric TBs in B vr BC, we identified a new type of asymmetric twin that constitutes the phase boundaries between boron rich boron carbide (B 13 C 2 ) and B vr BC (B 14 C). The predicted mechanical response of these asymmetric twins indicates a significant reduction of the ideal shear strength compared to single crystals B 13 C 2 and B 14 C, suggesting that the asymmetric twins facilitate the disintegration of icosahedral clusters under applied stress, which in turn leads to amorphous band formation and brittle failure. These results provide a mechanistic basis towards understating the roles of TBs in B vr BC and related superhard ceramics.
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Strengthening boron carbide through lithium dopant
Abstract The high strength of boron carbide (B4C) is essential in its engineering applications such as wear‐resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B4C can be enhanced by doping lithium to boron‐rich boron carbide (B13C2) to form r‐LiB13C2. The bonding analysis on r‐LiB13C2indicates that the electron counting rule (or Wade's rule) is satisfied in r‐LiB13C2whose formula can be written as r‐Li+(B12)2‐(CB+C). The shear deformation on r‐LiB13C2indicates that its ideal shear strength is larger than that of B4C because of the existing of Li dopant. The failure process of r‐LiB13C2under ideal shear deformation initiates from breaking the icosahedral‐icosahedral B‐B bonds. Then these B atoms react with the middle B in the C‐B‐C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r‐LiB13C2is even stronger than r‐LiB13C2because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r‐LiB13C2has a significant enhanced strength compared to B4C. Our simulation results illustrate the deformation mechanism of Li‐doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B13C2and increasing its twin densities.
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- Award ID(s):
- 1727428
- PAR ID:
- 10458769
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 103
- Issue:
- 3
- ISSN:
- 0002-7820
- Page Range / eLocation ID:
- p. 2012-2023
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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